1. Field of the Invention
The present invention relates to an ink excellent in recovery property of image performance when printing is started again after the ink in an ink cartridge has been mounted on an inkjet recording apparatus for a long time period, and to an ink set, an ink jet recording method, an ink cartridge, a recording unit, and an inkjet recording apparatus each using the ink.
2. Related Background Art
An ink jet recording method is a recording method of applying a small ink droplet to any one of recording media such as plain paper and glossy media to form an image, and has become rapidly wide spread owing to a reduction in its cost and an improvement in its recording rate. With the rapid spread of digital cameras in addition to an improvement in quality of an image recorded by the method, the method has been generally used as a method of outputting a photographic image comparable to silver halide photography.
In recent years, image quality has undergone an improvement never possible before owing to, for example, an extreme reduction in size of an ink droplet, an improvement of color gamut achieved by the introduction of multi-color inks, and the emergence of a coloring material capable of improving image fastness properties such as light resistance and gas resistance. Meanwhile, there have been growing loads on the molecular design of a coloring material and the design of an ink composition securing reliability requested for ink jet ink.
A large number of proposals concerning various kinds of coloring materials, inks, and ink set compositions have been made with a view to solving the above problems (see, for example, Japanese Patent Application Laid-Open No. 2004-2814).
The solubility of a coloring material needs to be optimized in seeking for such a coloring material as to enhance image performance and image fastness properties. Similarly, when one seeks for the composition of ink with which printing providing high image quality can be performed at an increased speed, the physical properties and the like of the ink needs to be optimized. Of course, coloring materials having different colors have different chemical structures, and even coloring materials having the same water-soluble group are different from each, other in solubility with respect to water or a water-soluble organic solvent. Therefore, for an ink for each color to be used for color recording, the ink composition optimized for the coloring material contained in the ink, has been developed. As a result, it can be said that the inks for respective colors are generally different from each other in ink composition.
In particular, in recent years, there has been growing performance requested for ink jet ink. Conventionally, inks containing different coloring materials but having the same composition have caused no problem. However, such inks may cause a problem when used for recent ink jet inks. Accordingly, inks for respective colors needs to be different from each other in ink composition.
In view of the above, the inventors of the present invention have mounted, on a recording head, an ink cartridge containing inks for respective colors each having a composition optimized for its coloring material so as to be applicable to an ink jet recording apparatus (such as an ink having a composition optimized for performances such as sticking recovery property from a state where the ink is left for a long time period without the use of a recording apparatus; or ejection stability of the ink in a low-temperature environment). Then, the inventors have performed printing again after an ink jet recording apparatus had been left for a relatively short time period (about 4 days). As a result, a phenomenon occurred, in which a portion printed at an early stage of printing became pale or concentrated depending on ink. As a result, in an image, a portion printed in the first half of printing (an upper portion when the image was observed from its front) caused color unevenness, so the image did not have any allowable image quality.
The inventors of the present invention have observed the image to find the following. The phenomenon occurs over the range from a fifth line (scanning 5 times) to about a tenth line (scanning 10 times) from the start of printing in an ink jet recording apparatus that performs recording by virtue of scan with a recording head, and then the phenomenon is gradually eliminated with printing. The inventors have also found that the phenomenon is particularly remarkable when in an ink for a certain color a specific water-soluble compound is added and in inks for the other colors such compound is not added because the compound is not needed for the performance of the inks for the other colors. The inventors have also found that the phenomenon is particularly remarkable in an ink jet recording apparatus having a constitution in which ejection orifices for two or more kinds of inks in a recording head are capped with one cap part.
To provide an image obtained through an ink jet recording method with image performance comparable to that of silver halide photography, it is most important to improve the color developability and image fastness properties of an ink for each color and to print an image capable of maintaining its performance regardless of a recording environment. In other words, such deterioration of the state of a printed image as described above is of great concern. Moreover, a phenomenon in which the use of inks for an ink set causes a problem in terms of performance such as image quality or reliability, even though each of the inks causes no problem in terms of performance such as image quality or reliability when the ink is singly evaluated for such performance, is a problem not conceived hitherto.
Therefore, an object of the present invention is to provide an ink excellent in recovery property of image performance when printing is started again after the ink in an ink cartridge has been mounted on an ink jet recording apparatus for a long time period.
Another object of the present invention is to provide an ink set, an ink jet recording method, an ink cartridge, a recording unit, and an ink jet recording apparatus each using the ink.
The above objects are achieved by the present invention described below.